1. Field of the Invention
The present invention relates to an electrophotographic carrier, a two-component developer comprising the carrier, and an electrophotographic image-forming process and apparatus using said developer. In particular, the invention relates to an electrophotographic carrier comprising a core particle and resin layers for coating the core particle, a two-component developer comprising the carrier, and an image-forming process and apparatus, such as copiers, printers, facsimile machines and the like, using said developer.
2. Description of the Related Art
In electrophotographic image-forming apparatuses, generally, an image is formed by the steps of charging, light-exposing, developing, transferring, cleaning, discharging and fixing. More specifically, for example, the surface of a rotating photoconductor drum is uniformly charged by a charger device, and then exposed to laser light emitted from a light-exposure device according to the image information, thereby forming an electrostatic latent image on the surface of the photoconductor drum. The latent mage is developed by a developing device into a toner image, which is then transferred by a transfer device onto a recording material, where the toner image is heated to be fixed by a fixing device. The residual toner on the surface of the photoconductor drum is removed off and collected in a collection chamber by a cleaning device. The cleaned surface of the photoconductor drum is discharged by a discharger device so as to be ready for the next round of the image-forming process.
For developing electrostatic latent images, generally single-component developers comprising toner alone, or two-component developers comprising toner and carriers are used.
Since single-component developers do not need to be stirred before use, they have an advantage that developing devices used therefor have a simple structure with no mixer or the like. However, they have a problem of being difficult to charge toner stably, etc.
On the other hand, since two-component developers need to be stirred before use in order to homogeneously mix toner and carrier, they have a problem that developing devices used therefor have a complicated structure with a mixer or the like. However, two-component developers have good charge stability and good applicability to high-speed machines, and therefore are commonly used in high-speed image-forming apparatuses and multicolor image-forming apparatuses.
As carriers used in two-component developers, magnetic particles of ferrite or the like having a particle size of 20 to 100 μm are generally used. It is known that magnetic particles are coated with acrylic resin, silicone resin or the like (referred to as “coated carriers”), in order to reduce the moisture-dependent changes of the characteristics and adhesion of a toner component onto the surfaces.
However, such coated carriers have high electrical resistance and therefore electric charge opposite in polarity to toner (i.e., counter-charge) tends to remain on their surfaces. Thus, they have a problem that, when used for developing solid images, the image density can be decreased.
In order to resolve this problem, Japanese Laid-Open Patent Publication No. Hei 2-309365 (1990) has proposed a coated carrier whose coating layer contains a photoconductive material. Since light-exposure to the coated carrier causes a decrease in the electrical resistance of the coating layer (carrier resistance), and thus a reduction in the counter-charge remaining on the surface, the coated carrier prevents from decreasing the image density when used for developing solid images.
However, such a coated carrier has a problem that the decreased carrier resistance makes it easier for electric charge to transfer from the photoconductor drum surface to the carrier so as to erase the latent image on the drum surface, and/or for the carrier to travel onto the drum surface so as to cause blurring or fogging in the formed images.